A problem exists in many systems when the various primary and related signals are processed (including by transmission and by storage, either direct or indirect, within a facility and/or to/from a facility). Examples include video systems wherein there are many video and/or audio signal sources from television cameras, tape recorders, direct feeds, remote feeds from microwave and satellite, not to mention what might occur to the signal--noise reduction, synchronizers, transcoders, switching facilities, obscenity drop circuits, etc.
The invention is specifically usable in respect to television signals and will be described with such signals for a preferred embodiment. For example, a problem exists in many television facilities when the various video and audio signals are processed within the station (or from facility to facility for that matter). There are also many video signal sources; from television cameras, video tape recorders, remote feeds from microwave and satellite, together with signal manipulations such as noise reduction, etc. There are also many audio signal sources; from microphones, tape recorders, remote feeds, etc. subject to their own similar processing.
Television facilities utilize routing switchers in order that the video and audio from any source may be connected to the input of any video or audio device respectively, with the outputs of those devices also being connected to the router so that the video and audio can be connected to the input of any other instrument. The video and audio may, and frequently do, take entirely different paths having entirely different delays. These paths can include many forms of active and/or passive delays.
It is not uncommon to have a 64 by 64 matrix for each of the auxiliary and video signals. The output signal is frequently changed in delay by selection of different signal processing paths and selection of different ones of the input signals to be output. These router systems give literally hundreds of possibilities for signal path combinations, with the paths being frequently changed to facilitate operational needs, differing network feeds, local feeds, differing active processing circuits, et al.
Frequently, the video processing path involves several devices which can have one or more frames of delay. Many of these devices will have a changing delay as the mode of operation is changed by the operator, as the phase of the incoming video signal drifts with respect to the other video signals or to the systems reference, or otherwise. Examples include noise reduction systems, synchronizers, transcoders, drop out compensators et al.
It is not unusual for the video to suffer delays ranging from near zero to 10 frames of delay or more--i.e. .32 seconds before the signal exits the system.
The audio and secondary signals on the other hand usually are passed through devices which have relatively little delay compared to the video (although delays may be present due to processing similar to the video outline above). Frequently there are several audio channels, for example 2 channels of English, 2 channels of another language, a data channel and a control channel. However, when the video signal and the auxiliary signals experience relative timing variations due to the changing delays of the different paths, problems occur, the most commonly noticed one being lip sync error when the video is delayed with respect to the audio.
Variable delay devices exist for delaying signals and it is possible at any point in the system to delay the earlier arriving (typically audio) signal(s) to match the later arriving (typically video) signal. However, the problem in making such corrections arises in detecting the delay of the later (typically video) signal which is output from the system. This problem is compounded because the relative delay is constantly changing: the delay is often instantly changed as a signal is routed through different processing devices, and different ones of the many input signals are selected to be passed to the output.
The U.S. Pat. No. 5,202,761, Audio Synchronization Apparatus, is an attempt to solve some of these related problems. In this '761 patent, a pulse was added only in response to the video signal and then only in the vertical interval. It therefore would wait until the next vertical interval to add a pulse in the vertical interval. This would function within a certain given accuracy. Since all of the video inputs may be totally asynchronous some video signals might have their vertical interval pulse added immediately. However, others might have to wait nearly a whole frame to have their pulse added. This gives rise to an inherent one frame inaccuracy. Further, various devices exist which over write or remove the vertical interval, severely compromising the '761's performance.
These problems are not true in the present invention. In this invention, the delay of the signals which pass through the systems are measured through the use of a delay tracker which is associated with the signal. This delay tracker is carried with the signal through the system so that, when the selected one of the possibly many input signals is output from the system, the presence of the delay tracker can be detected for that (those) signal(s) by themselves or in combination with other signals. In a preferred embodiment, a delay measurement circuit means receives the delay tracker (directly or indirectly as later set forth) and starts counting time. When the delay tracker is detected on the signal out of the system, a delay tracker detected signal is sent to the delay measurement circuit which then stops the time count. The time at the stop is thereby a measure of the delay of the signal through the system. This measure of delay can then subsequently used, for example to adjust the synchronization of the various signals or otherwise.